of Sciences and Technology, 305- 701 Taejon, Republic of Korea ap University of Alabama, Tuscaloosa, AL 35486, USA aq Purdue University, West Lafayette, IN 47907, USA ar Paul Scherrer
Trang 1North-Holland
Search for lepton flavour violation in Z decays
L3 Collaboration
O Adriani o, M Aguilar-Benitez x, S Ahlen i, j Alcaraz P, A Aloisio aa, G AlversonJ,
M.G Alviggi aa, G Ambrosi af, Q An q, H A n d e r h u b at, A.L Anderson n, V.P Andreev aj,
T Angelescu k, L A n t o n o v an, D Antreasyan g, P Arce x, A Arefiev z, A A t a m a n c h u k aj,
T A z e m o o n c, T Aziz h, P.V.K.S Baba q, P Bagnaia ai, J.A Bakken ah, R.C Ball c, S Banerjee h,
J Bao e, R Barillbre p, L Barone ai, A Baschirotto Y, R Battiston af, A Bay r, F Becattini o
J Bechtluft a, R Becker a, U Becket n'at, F Behner at, J Behrens at, Gy.L Bencze e, J Berdugo x,
P Berges n, B Bertucci af, B.L Betev an,at, M Biasini af, A Biland at, G.M Bilei af, R Bizzarri ai, J.J Blaising d, G.J Bobbink p,b, R Bock a, A B 6 h m a, B Borgia ai, M Bosetti Y, D Bourilkov ac,
M B o u r q u i n r, D Boutigny v, B Bouwens b, E Brambilla aa, J.G Branson ak, I.C Brock as,
M Brooks v, A Bujak aq, J.D Burger n, W.J Burger r, J Busenitz ap, A Buytenhuijs ac, X.D Caiq,
M Capell n, M Caria af, G Carlino aa, A.M Cartacci o, R Castello Y, M Cerrada x, F Cesaroni ai, Y.H C h a n g n, U.K C h a t u r v e d i q, M C h e m a r i n w, A Chen av C Chen f, G Chen f, G.M Chert f, H.F Chert s, H.S Chen f, M C h e n n, W.Y Chen av, G Chiefari aa, C.Y Chien e, M.T Choi ao,
S C h u n g n, C Civinini °, I Clare n, R Clare n, T.E Coan v, H.O C o h n aa, G Coignet d,
N Colino P, A Contin g, F Cotorobai k X.T Cui q, X.Y Cui q, T.S Dai ", R D'Alessandro o,
R de A s m u n d i s aa, A Degrd d, K Deiters ar, E D6nes e, P Denes ah, F DeNotaristefani ai,
M D h i n a , D DiBitonto , M D i e m o z , H.R D i m i t r o v , C Dionisi ai, M D i t t m a r at,
L D j a m b a z o v at, M.T D o v a q, E Drago aa, D D u c h e s n e a u r, P Duinker b, I D u r a n a~, S Easo af,
H El M a m o u n i w, A Engler as, F.J Eppling n, F.C Ern6 b, P E x t e r m a n n r, R Fabbretti at,
M Fabre at, S Falciano ai, S.J Fan am, O Fackler u, J Fay w, M Felcini P, T Ferguson ag,
D F e r n a n d e z x, G F e r n a n d e z x, F Ferroni ai, H Fesefeldt a, E Fiandrini af, J.H Field r,
F Filtbaut ac, P.H Fisher e, G Forconi r, L Fredj r, K Freudenreich at, W Friebel as,
M F u k u s h i m a n, M Gailloud t, Yu G a l a k t i o n o v z,n, E Gallo o, S.N Ganguli p,h, p Garcia-Abia x,
D Gele w, S Gentile ai, N G h e o r d a n e s c u k, S Giagu ai, S Goldfarb j, Z.F G o n g s, E Gonzalez x,
A Gougas e, D G o u j o n r, G Gratta ae, M G r u e n e w a l d p, C Gu q, M Guanziroli q, J.K G u o am, V.K G u p t a ab, A G u r t u h, H.R Gustafson c, L.J G u t a y aq, K Hangarter a, B H a r t m a n n a,
A Hasan q, D Hauschildt b, C.F He am, J.T He f, T Hebbeker p, M Hebert ak, A Herv6 p,
K Hilgers a, H Hofer at, H Hoorani r, G Hu q, G.Q Hu am B Ille w, M.M Ilyas q, V Innocente P,
H Janssen P, S Jezequel d, B.N Jin f, L.W Jones c, I Josa-Mutuberria p, A Kasser t,
R.A K h a n q, Yu K a m y s h k o v ad p Kapinos aj,as, J.S Kapustinsky v, y Karyotakis P, M Kaur q,
S K h o k h a r q, M.N Kienzle-Focacci r, J.K K i m a°, S.C Kim a°, Y.G Kim a°, W.W Kinnison v,
A Kirkby ae, D Kirkby ae, S Kirsch as, W Kittel ac, A K l i m e n t o v n,z, R Kl6ckner a,
A.C K6nig ac, E K o f f e m a n b, O K o r n a d t a, V K o u t s e n k o n,z, A Koulbardis aj, R.W K r a e m e r as,
T K r a m e r n, V.R Krastev an,af, W Krenz a, A Krivshich aj, H Kuijten ac, K.S K u m a r m,
A K u n i n n,z, G Landi o, D Lanske a, S Lanzano aa, A Lebedev n, p Lebrun w, p Lecomte at,
P Lecoq p, P Le Coultre at, D.M Lee v, J.S Lee a°, K.Y Lee a°, I L e e d o m J, C Leggett c,
J.M Le G o l f p, R Leiste as, M Lenti °, E Leonardi ai, C Li s'q, H.T Li f, P.J Li am, J.Y Liao am, W.T Lin av, Z.Y Lin s, F.L Linde b, B L i n d e m a n n a, L Lista aa, Y Liu q, W L o h m a n n as,
E Longo ai, Y.S Lu f, J.M Lubbers p, K Liibelsmeyer a, C Luci ai, D Luckey g,n, L Ludovici ai,
Trang 2L L u m i n a r i ai, W L u s t e r m a n n as, J.M Ma f, W.G Ma s, M M a c D e r m o t t at, R Malik q,
A Malinin z, C Mafia x, M Maolinbay at, p Marchesini at, F Marion d, A Marin i, j.p Martin w,
L Martinez-Laso x, F Marzano ai, G.G.G Massaro b, K M a z u m d a r r, P McBride m,
T M c M a h o n aq, D McNally at, M Merk aLL Merola aa, M Meschini °, W.J Metzger ac, Y Mi t,
A Mihul k, G.B Mills v, Y Mirq, G Mirabelli ai, J Mnich a, M M611er a, B Monteleoni °,
R M o r a n d d, S Morganti ai, N.E Moulai q, R M o u n t ae, S Miiller a, A N a d t o c h y aj, E Nagy t,
M Napolitano aa, F Nessi-Tedaldi at, H N e w m a n ae, C Neyer at, M.A Niaz q, A Nippe a,
H Nowak as, G Organtini ai, D Pandoulas a, S Paoletti ai, p Paolucci aa, G Pascale ai,
G Passaleva o,af, S Patricelli aa, T Paul e, M Pauluzzi af, C Paus a, F Pauss at, y j Pei a,
S Pensotti Y, D Perret-Gallix d, J Perrier r, A Pevsner e, D Piccolo aa, M Pieri v, P.A Pirou6 ah,
F Plasil aa, V Plyaskin z, M Pohl at, V Pojidaev z,°, H P o s t e m a n, Z.D Qi am, J.M Qian c, K.N Qureshi q, R Raghavan h, G Rahal-Callot at, P.G RancoitaY, M Rattaggi Y, G Raven b,
P Razis ab, K Read ad, D Ren at, Z Ren q, M Rescigno ai, S Reucroft J, A Ricker a, S R i e m a n n as, B.C Riemers aq, K Riles c, O R i n d c, H.A Rizvi q, S Ro a°, F.J Rodriguez x, B.P Roe c,
M R 6 h n e r a, L R o m e r o x, S Rosier-Lees d, R R o s m a l e n ac, Ph Rosselet t, W van R o s s u m b,
S R o t h a, A Rubbia n, J.A Rubio p, H Rykaczewski at, M Sachwitz as, J Salicio p, J.M Salicio x, G.S Sanders v, A Santocchia af, M.S Sarakinos", G Sartorelli s,q, M Sassowsky a, G Sauvage d,
V Schegelsky aj, D Schmitz a, p Schmitz a, M Schneegans d, H Schopper au, D.J Schotanus ac,
S Shotkin n, H.J Schreiber as, J Shukla aL R Schulte a, S Schulte a, K Schultze a, J Schwenke a,
G Schwering a, C Sciacca aa, I Scott m, R Sehgal q, P.G Seiler at, J.C Sens p,b, L Servoli af
I Sheer ak, D.Z Shen am, S Shevchenko ae, X.R Shi ae, E Shumilov z, V Shoutko z, D Son ao,
A Sopczak P, V Soulimov aa, C Spartiotis e, T S p i c k e r m a n n a, p Spillantini o, R Starosta a,
M SteuerS.n, D.P Stickland ah, F Sticozzi n, H Stone ah, K Strauch m, B.C Stringfellow aq,
K Sudhakar h, G Sultanov q, L.Z Sun s,q, G.F Susinno r, H Suter at, J.D Swain q, A.A Syed ac, X.W Tang f, L TaylorJ, G Terzi Y, Samuel C.C Ting n, S.M Ting n, M Tonutti a, S.C T o n w a r h,
J T6th t, A Tsaregorodtsev aj, G Tsipolitis ag, C Tully ah, K.L T u n g f, J Ulbricht at, L Urbfin t,
U U w e r a, E Valente ai, R.T Van de Walle ac, I Vetlitsky z, G Viertel at, p Vikas q, U Vikas q,
M Vivargent d, H Vogel aL H Vogt as, I Vorobiev z, A.A Vorobyov aj, L Vuilleumier t,
M W a d h w a d, W Wallraff a, C Wang n, C.R Wang s, X.L Wang s, Y.F Wang n, Z.M Wang q,s,
C Warner a, A Weber a, j Weber at, R Weill t, T.J Wenaus u, j Wenninger r, M White n,
C Willmott x, F Wittgenstein p, D Wright ah, S.X Wu q, S W y n h o f f a, B Wystouch n,
Y.Y Xie am, J.G Xu f, Z.Z Xu s, Z.L Xue ara, D.S Yan am, B.Z Yang s, C.G Yang f, G Yang q, C.H Ye q, J.B Ye s, Q Ye q, S.C Yeh av, Z.W Yin am, J.M You q, N Y u n u s q, M Y z e r m a n b,
C Zaccardelli ae, N Zaitsev aa, P Z e m p at, M Zeng q, Y Zeng a, D.H Zhang b, Z.P Zhang s'q,
B Z h o u i, G.J Z h o u f, J.F Z h o u a, R.Y Zhu ae, A Zichichi g'P'q and B.C.C van der Zwaan b
a L Physikalisches lnstitut, R W T H , W-5100 Aachen, FRG I
IlL Physikalisches lnstitut, R WTH, W-5100 Aachen, FRG l
b National Institute for High Energy Physics, NIKHEF, NL-IO09 DB Amsterdam, The Netherlands
c University o f Michigan, Ann Arbor, M I 48109, USA
d Laboratoire d'Annecy-le-Vieux de Physique des Particules, LAPP, IN2P3-CNRS B.P 110, F-74941 Annecy-le-Vieux Cedex, France
e Johns Hopkins University, Baltimore, MD 21218, USA
f Institute of High Energy Physics, IHEP, 100039 Beijing, China
g INFN - Sezione di Bologna, 1-40126 Bologna, Italy
h Tata Institute of Fundamental Research, Bombay 400 005, India
i Boston University, Boston, MA 02215, USA
J Northeastern University, Boston, MA 02115, USA
k Institute o f Atomic Physics and University of Bucharest, R-76900 Bucharest, Romania
t Central Research Institute for Physics o f the Hungarian Academy of Sciences, H-1525 Budapest 114, Hungary 2
Trang 3m Harvard University, Cambridge, MA 02139, USA
n Massachusetts Institute of Technology, Cambridge, MA 02139, USA
o INFN - Sezione di Firenze and University of Florence, 1-50125 Florence, Italy
P European Laboratory for Particle Physics, CERN, CH-1211 Geneva 23, Switzerland
q World Laboratory, FBLJA Project, CH-1211 Geneva 23, Switzerland
r University of Geneva, CH-1211 Geneva 4, Switzerland
s Chinese University of Science and Technology, USTC, Hefei, Anhui 230 029, China
t University of Lausanne, CH-1015 Lausanne, Switzerland
u Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
v Los Alamos National Laboratory, Los Alamos, NM 87544, USA
w Institut de Physique NuclJaire de Lyon, IN2P3-CNRS, UniversitJ Claude Bernard F-69622 Villeurbanne Cedex, France
x Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas, CIEMAT, E-28040 Madrid, Spain
Y INFN - Sezione di Milano, 1-20133 Milan, Italy
z Institute of Theoretical and Experimental Physics, ITEP, Moscow, Russia
a a 1NFN - Sezione di Napoli and University of Naples, 1-80125 Naples, Italy
ab Department of Natural Sciences, University of Cyprus, Nicosia, Cyprus
a¢ University of Nymegen and NIKHEE NL-6525 ED Nymegen, The Netherlands
ad Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
a e California Institute o f Technology, Pasadena, CA 91125, USA
a f INFN - Sezione di Perugia and Universitiz Degli Studi di Perugia, 1-06100 Perugia, Italy
as Carnegte Mellon University, Pittsburgh, PA 15213, USA
ah Princeton University, Princeton, NJ 08544, USA
ai INFN - Sezione di Roma and University of Rome, "La Sapienza", 1-00185 Rome, Italy
aj Nuclear Physics Institute, St Petersburg, Russia
ak University of California, San Diego, CA 92093, USA
at Dept de Fisica de Particulas Elementales, Univ de Santiago, E-15706 Santiago de Compostela, Spain
am Shanghai Institute of Ceramics, SIC, ShanghaL China
an Bulgarian Academy of Sciences, Institute of Mechatronics, BU-I 113 Sofia, Bulgaria
ao Center for High Energy Physics, Korea Advanced Inst of Sciences and Technology, 305- 701 Taejon, Republic of Korea
ap University of Alabama, Tuscaloosa, AL 35486, USA
aq Purdue University, West Lafayette, IN 47907, USA
ar Paul Scherrer Institut, PSI, CH-5232 Villigen, Switzerland
as D E S Y - Institut fiir Hochenergiephysik, 0-1615 Zeuthen, FRG
at Eidgen6ssische Technische Hochschule, ETH Ziirich, CH-8093 Zarich, Switzerland
au University of Hamburg, W-2000 Hamburg, FRG
av High Energy Physics Group, Taiwan, China
Received 9 August 1993
Editor: K Winter
We have searched for lepton flavour violation in Z boson decays into lepton pairs using all data collected with the L3 detector during the 1990, 1991 and 1992 runs on an event sample corresponding to 1 500 000 2?s produced At the 95% confidence level
the upper limits on the branching ratio for Z ,etz is 0.6X 10 -s, for Z .er this is 1.3X 10 -5 and for Z */zT this is 1.9X 10 -s
1 I n t r o d u c t i o n
I n the S t a n d a r d Model [ 1,2 ] lepton flavour is c o n -
1 Supported by the German Bundesministerium t'dr
Forschung und Technologie
2 Supported by the Hungarian OTKA fund under contract
number 2970
served However, there is no gauge principle requir- ing this conservation Different models [ 3 - 9 ], be-
y o n d the S t a n d a r d Model, allow processes which
v i o l a t e lepton flavour conservation In theories where such v i o l a t i o n arises through m i x i n g with new p a n i - cles [4,6], the b r a n c h i n g ratios for such processes, e.g Z ./zr, have been calculated to be as large as 10 -4
in certain models T h e o b s e r v a t i o n o f such decays
Trang 4would be a clear indication of physics beyond the
Standard Model Here a search for lepton flavour vi-
olation in Z decays into e/t, ez and/tT is reported us-
ing the data collected with the L3 detector during the
1990, 1991 and 1992 runs on an event sample corre-
sponding to l 500 000 Z's produced Direct searches
for lepton flavour violation [ 10-17 ] have been per-
formed previously by L3 and other experiments
Stringent constraints on violation o f / t flavour exist
from low-q 2 reactions, such as the absence of the de-
cay/t ,eee [ 14 ], providing an upper limit [ 18 ] on
Br ( Z * e/t) of 6.6 × 10- ~ 3 Searches for neutrinoless
r decays r ~ e e e , z ./t/t/t [12,13] lead to much less
stringent limits on Br(Z .er) and Br(Z ./tr)
while retaining a high detection efficiency The en- ergy resolution for electrons determined from Bhabha events is 1.3% at 45 GeV, while the muon momen- tum resolution determined from dimuon events is 2.5%
About 300 000 Monte Carlo events were generated
to study Standard Model backgrounds from e ÷ e - , /t+/t-, r ÷ z - and four-fermion channels using var- ious Monte Carlo generators [20-23] A modified version of the KORALZ [20] Monte Carlo program was used to generate signal events
4 Preselection and lepton identification
2 The 1.3 detector
The fiducial solid angle of the L3 detector [ 19 ] is
99% of 4n The detector consists of a time expansion
chamber (TEC) for tracking charged particles, a high
resolution electromagnetic calorimeter of BGO crys-
tals, a barrel of scintillation counters, a hadron calo-
rimeter with uranium absorber and proportional wire
chamber readout and a muon spectrometer The lu-
minosity is determined from small-angle Bhabha
scattering using BGO electromagnetic calorimeters in
the polar angle ranges 0 and n - 0 between 24.93 and
69.94 mrad All subdetectors are installed inside a 12
m diameter solenoidal magnet which provides a uni-
form 0.5 T field along the beam direction A detailed
description of each detector subsystem, and its per-
formance, is given in refs [ 11,19 ]
3 Signature and background
The expected experimental signature of Z .eg,
Z - , / t r and Z .er is an electron or muon with an en-
ergy close to the beam energy recoiling against a dif-
ferent type of lepton or hadrons from tau decay The
background arises from Standard Model leptonic fi-
nal states and can be divided into two classes: (i) in-
correctly reconstructed e + e - and g + / t - events; (ii)
r+T - events with one or both of the taus decaying
into a moon or electron which carries almost all the
energy of the tau Good muon and electron energy
resolution is essential to reduce the latter background
The preselection cuts, used to select a data sample containing high energy dilepton events of all types, are the following:
- The total energy is greater than 30 GeV
- The number of jets is 2
- The number of tracks in the TEC is between 1 and 6, to remove hadron events
- The number of calorimeter clusters is less than
15, to remove hadron events
- The acolinearity angle between the two jets is smaller than 20 °, to remove radiative events Jets are reconstructed using a two step algorithm [24] which groups the energy deposited in calorim- eters into clusters before collecting the clusters into jets The clustering algorithm normally reconstructs one cluster for each muon, electron or photon shower, and a few clusters for a hadronic decay of a single z Under the above definition of a jet, particles with only one cluster, like electrons, are also considered as jets
An electron is defined as a geometrical cluster in the electromagnetic calorimeter with an energy larger than 2 GeV matched with a TEC track in the (R, ¢) plane within l0 mrad The cluster shower profile should be consistent with that of an electron, i.e we require 0.97<Eg/E25<l.025, where E9(25) is the corrected sum of energies of 9(25) BGO crystals around the most energetic one, and X 2 < 2, where X 2
is obtained by fitting the shape of the cluster to the mean electron shower profile The electron candidate must be in the fiducial volume defined by Icos01 <0.9
Muons are identified and their m o m e n t a mea- sured in the muon chamber system surrounding the
Trang 5calorimeters To be accepted, a muon track must have
one track segment in each of the three chamber lay-
ers The accepted muon track must extrapolate back
to within 100 mm of the nominal vertex position in
the transverse plane and 200 m m in the longitudinal
plane The muon candidate energy must be greater
than 3 GeV and it must be in the fiducial volume de-
fined by ]cos 0] < 0.75
5 Z ~ e t i c h a n n e l
For the Z ~ e l * channel we require one jet to be con-
sistent with a beam energy electron and the other one
to be consistent with a beam energy muon This type
of event is essentially free of background and allows
a less restrictive selection cut on lepton energy than
ised electron energy versus the normalised muon mo-
mentum for the data events selected Signal events
will populate the region where both normalised ener-
gies are close to unity We require that for the elec-
tron candidate the normalised energy should be
greater than 0.96 ( ~ 30) and for the muon candidate
greater than 0.92 ( ~ 30) No events lie within this
region giving the 95% confidence level limit on the
1.05
0 95
.8
U /~= 085
W
0.75
Q
I
0 % ~ 5 , 0.75 , • 0 85 , 0.95 1.05
Ee/Ebeam
candidate events The shaded area represents the region corre-
sponding to the signal events
signal of three events We apply the above cuts to the sample of the signal and Standard Model background (z+z -, e+e - and ~ + # - ) Monte Carlo events We find no events in the Standard Model Monte Carlo sample The efficiency of the analysis as determined
by the signal Monte Carlo is (32.0_+ 1.0)% This er- ror includes the uncertainty due to the Monte Carlo statistics The additional systematic error due to the corrected number of hadronic Z decays used for nor- malization and due to the uncertainties in the detec- tor calibration constants is estimated to be 1.5% [25 ] This yields a 95% confidence level limit on the branching ratio of
Br(Z .e/t) < 0.6 X 10 -5
6 Z - - * el" c h a n n e l
For Z - , er we require one jet to be consistent with
a beam energy electron and the other one to be con- sistent with a z decay We consider the following z decays:
muon with an energy greater than 3 GeV
electron with an energy less than 30 GeV The had- tonic energy, contained in the z jet, should be less than 0.5 GeV To reject the Z - , e e ( 7 ) background we re- quire that the acoplanarity angle between TEC tracks associated with the electron and the z be more than 3.2 mrad Fig 2 shows the distribution of this vari- able for Z - , e e ( 7 ) as well as Z .er events
- z-, 1 prong We require that the z-jet, containing only one TEC track, has a total energy larger than 3 GeV and electromagnetic energy less than 30 GeV The hadronic energy contained in the z jet should be greater than 0.5 GeV To reject Z - , e e ( 7 ) events, where one electron passes close to the cracks in the electromagnetic calorimeter and therefore deposits some energy in the hadron calorimeter, we require that the energy in the hadron calorimeter beyond its first 25Xo be greater than 10% of the total jet energy The above cut on the acoplanarity angle between two TEC tracks is also applied
- z-.3 or 5 prongs We require that the r-jet con- tains at least two TEC tracks, has a total energy larger than 3 GeV, has an electromagnetic energy less than
Trang 61 0 d
~ , • Data [ i Z° )eev MC 10: # ~ i ! Z ° ~ e : MC
I I
I i/i
(Sq:) ( r a d )
Fig 2 The distribution of acoplanarity ansle between two oppo-
site TEC tracks in the dflepton events The points are data, the
histogram corresponds to the Z-*ee(7) Monte Carlo events and
the dashed line shows the distribution for the signal Z ,er Monte
Carlo events The normalisation for the signal distribution is ar-
bitrary The arrows indicate the cuts
30 GeV a n d an energy deposition in the h a d r o n cal-
orimeter larger t h a n 1 GeV To r e m o v e four-fermion
events, which have no missing energy, we require that
energy in the h a d r o n i c calorimeter be less t h a n
0.85 (Ebe=m EECAL)
Fig 3 shows the d i s t r i b u t i o n o f the electron energy
after all cuts b u t the electron energy cut have been
applied To reject the r e m a i n i n g Z - - , r r b a c k g r o u n d
we require that the energy of the electron should be
larger than 0 9 8 5 E ~ m After all cuts this yields a n
efficiency for the Z , er c h a n n e l of (23.8 _+ 0.9 ) %
Table 1 shows the estimated acceptance for the
Z - , e r c h a n n e l for the different r decay modes to-
gether with the n u m b e r o f surviving events after all
C u t s
We find two events in the data, while we expect
2.8 + 0.7 events from Z ,rz a n d Z ,ffff Monte Carlo
We set a 95% CL u p p e r limit of 4.7 events for the
Z - - e r channel, yielding a 95% CL limit on the
b r a n c h i n g ratio of
B r ( Z - - , e r ) < 1.3X 10 -5
40
t-
>
W
• D a t a [ J Z ° ~ f f f f MC
3O I I
Cut
ii
, - - - , '-';'": """ i
• 855 0.905" " 0.955 ' 1005 '1.055
Ee/Ebeam
Fig 3 The distribution of electron energy after all cuts but the electron energy cut have been applied in the Z er selection
Table 1 Acceptance, expected background events and number of events seen in the data for the Z , er channel for different r decay modes Channel Acceptance (%) Expected Seen
r ,l prong 8.5_+0.5 1.1_+0.4 2 r ,3 or 5 prongs 4.2_+0.4 1.0_+0.3 0
7 Z ,/t¢ channel
For Z - / z x we require one j e t to be consistent with
a beam energy m u o n a n d the other one to be consis- tent with a r decay We consider the following r decays:
- r - , # v O D u e to the b a c k g r o u n d from d i m u o n events, this decay c h a n n e l is n o t considered
- z - , e v a We require that the z-jet consists of one electron with energy greater t h a n 5 GeV T h e had- ronic energy, contained in the r jet, should be less than 0.5 GeV To reject the Z .#/z(y) b a c k g r o u n d we re- quire: ( i ) the acoplanarity between T E C tracks as-
Trang 7s o c i a t e d with the m u o n a n d the r to be greater t h a n
3.2 mrad; ( i i ) there are no track segments in the m u o n
c h a m b e r s in the h e m i s p h e r e o p p o s i t e to the /z
c a n d i d a t e
- r , 1 prong We require that the r-jet contains only
one T E C track, has a total energy larger t h a n 5 G e V
a n d an e l e c t r o m a g n e t i c energy less t h a n 35 GeV T h e
h a d r o n i c energy c o n t a i n e d in the r j e t s h o u l d be
greater t h a n 0.5 GeV To reject the Z ,Izlz(7) back-
ground, the slower profile o f the r j e t in the h a d r o n
c a l o r i m e t e r s h o u l d be i n c o n s i s t e n t with t h a t o f a
m u o n [ 10] Cuts ( i ) a n d ( i i ) f r o m the p r e v i o u s de-
cay channel are also applied
- r ,3 o r 5 prongs We require that the r-jet con-
tains at least two T E C tracks a n d has a total energy
greater t h a n 5 GeV T h e r e m u s t be no t r a c k segments
in m u o n c h a m b e r s in the h e m i s p h e r e o p p o s i t e to the
/z c a n d i d a t e
Fig 4 shows the d i s t r i b u t i o n o f the m u o n energy
after all cuts b u t the electron energy cut h a v e been
applied To reject the r e m a i n i n g Z .rr b a c k g r o u n d
we require t h a t the energy o f the m u o n s h o u l d be
larger t h a n 0.97E~am After all cuts this yields an ef-
ficiency for the Z ~ # r channel o f (22.0 + 0.9)%
Table 2 shows the e s t i m a t e d a c c e p t a n c e for the
Z ,Izr channel for the different r d e c a y m o d e s to-
50
Data ]
I • I Z ° ~ f f f f MC [
I I Z°~'t't.#ja M C I
[iiilSi Z°-~#T MC
Cut
40
Cut
U I
0 7 J "" , ' r ,
EJEbeam Fig 4 The distribution of muon energy after all cuts but the muon
energy cut have been applied in the Z-~/~r selection
Table 2 Acceptance, expected background events and number of events
seen in the data for the Z~/zr channel for different r decay modes Channel Acceptance (%) Expected Seen
r ,l prong I 1.8_+0.6 3.4_+0.8 3 T ,3 or 5 prongs 5.2_+0.5 0.7_+0.3 I
gether with the n u m b e r o f surviving events after all cuts
We f i n d 5 events in data, while we expect 5.9 + 1.2 events f r o m Z ~ r r , Z ~ / z / z a n d Z - - , f f f f M o n t e Carlo
We set a 95% C L u p p e r l i m i t o f 6.2 events for the
Z ~ / z r channel, yielding a 95% C L l i m i t on the
b r a n c h i n g ratio o f
B r ( Z - - / t r ) < 1 9 × 10 - 5
8 C o n c l u s i o n
We have searched for l e p t o n f l a v o u r v i o l a t i n g de- cays o f the Z boson, o b t a i n i n g the l i m i t s
B r ( Z ~ e / z ) < 0 6 X 10 -5 ,
B r ( Z - - , e r ) < 1 3 × 10 - 5 ,
B r ( Z - , / z z ) < 1.9X 10 - s These limits are a p p r o x i m a t e l y 5 t i m e s b e t t e r t h a n
p r e v i o u s l y p u b l i s h e d LEP results [ 15-17 ]
A c k n o w l e d g e m e n t
We wish to express o u r g r a t i t u d e to the C E R N ac- celerator d i v i s i o n s for the excellent p e r f o r m a n c e o f the LEP machine We acknowledge the efforts o f all engineers a n d t e c h n i c i a n s who have p a r t i c i p a t e d in the construction and m a i n t e n a n c e o f this experiment
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